/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineExternalUIInterface.h
Display the platform UI for sending a mailbox message to another user
true if it was able to show the UI, false if it failed
bool ShowSendMessageUI ( int32 LocalUserNum, const [FShowSendMessageParams](API\Plugins\OnlineSubsystem\Interfaces\FShowSendMessageParams) & ShowParams, const [FOnShowSendMessageUIClosedDelegate](API\Plugins\OnlineSubsystem\Interfaces\FOnShowSendMessageUIClosedDelega-) & Delegate )